In recent years, various wireless communication systems have adopted Orthogonal Frequency Division Multiplexing (OFDM). In this OFDM, since parallel transmission using a plurality of carriers is performed, the length of an individual symbol can be prolonged. Thus, even on a multipath communication channel having frequency selectivity, an individual received signal can be equalized by using a simple receiver configuration. Generally, in OFDM, to cope with a multipath communication channel, a cyclic prefix (CP) is arranged between OFDM symbols. Usually, the length of a CP is designed in view of a delay spread of a multipath communication channel. If the delay spread exceeds the CP length, OFDM Inter Symbol Interference (ISI) is caused in which OFDM symbols arranged before and after a desired symbol in the time direction interfere with the desired symbol, and Inter Carrier Interference (ICI) is caused in which sub-carriers in the frequency direction interfere with each other. When these ISI and ICI are caused, reception characteristics are deteriorated. ISI is caused when an OFDM symbol arranged before or after another OFDM symbol on which the receiver side performs Fast Fourier Transform (FFT) exceeds the CP length and leaks into the target FFT window. ICI is caused between sub-carriers when a delay spread exceeds the CP length and a channel matrix results in a non-circulant matrix and fails to be diagonalized by FFT.
As techniques to solve the above problems, receiving apparatuses disclosed in Patent Literatures (PTLs) 1 and 2 are known. PTLs 1 and 2 disclose examples in which, in an environment where a delay spread could exceed a guard interval (GI) corresponding to the length of a CP, a timing at which the signal-to-interference power ratio (SIR) in an FFT window reaches a maximum level on the receiver side is set as an FFT window start timing.
FIG. 12 is a block diagram illustrating detailed constituent elements relating to FFT window timing determination for describing symbol synchronization timing detection processing performed by the receiving apparatus disclosed in PTL 1. FIG. 12 illustrates a configuration including a window start timing candidate determination unit 1001, an intra-FFT-window signal power calculation unit 1002, an intra-FFT-window interference power calculation unit 1003, an intra-FFT-window SIR calculation unit 1004, and a maximum SIR timing detection unit 1005.
The window start timing candidate determination unit 1001 refers to a delay profile of a communication channel between transmitting and receiving apparatuses and outputs the beginning of an individual OFDM symbol or the end of an individual CP to the intra-FFT-window signal power calculation unit 1002 and the intra-FFT-window interference power calculation unit 1003 as a window start timing candidate. [Translation notes: The term “intra-FFT-window” denotes within a FFT window and not a between FFT windows”]
The intra-FFT-window signal power calculation unit 1002 calculates intra-FFT-window signal power by referring to the delay profile at an individual one of the window start timing candidates and outputs the calculated signal power to the intra-FFT-window SIR calculation unit 1004.
The intra-FFT-window interference power calculation unit 1003 calculates intra-FFT-window interference power by referring to the delay profile at the individual one of the window start timing candidates and outputs the calculated interference power to the intra-FFT-window SIR calculation unit 1004.
The intra-FFT-window SIR calculation unit 1004 calculates intra-FFT-window SIR by referring to the intra-FFT-window signal power and the intra-FFT-window interference power at the individual one of the window start timing candidates and outputs the calculated SIRs to the maximum SIR timing detection unit 1005.
The maximum SIR timing detection unit 1005 selects a window start timing achieving the maximum intra-FFT-window SIR from the window start timing candidates and outputs the selected window start timing.
FIG. 13 is a schematic diagram illustrating a relationship between the delay profile and the FFT window start timing candidates. As illustrated in a) in FIG. 13 [FIG. 13A], in conventional OFDM, the end of a CP of the first path is used as the FFT window start timing. In contrast, as illustrated in b) [FIG. 13B], in PTL 1, the beginning of an individual OFDM symbol and the end of a CP of an individual path are selected as the FFT window start timing candidate(s), and a window start timing is selected from the FFT window start timing candidates. The length of an FFT window is the same as the length of Inverse Fast Fourier Transform (IFFT), which is processing performed on the transmitter side of the OFDM transmission. Thus, a window end timing is determined by adding time period equivalent to the length of the transmission IFFT to the window start timing.
As described above, to improve its reception characteristics, the receiving apparatus disclosed in PTL 1 selects an FFT start window timing that achieves the maximum intra-FFT-window SIR.
As another technique to solve the same problems, a receiving apparatus disclosed in Non-Patent Literature (NPL) 1 is known. NPL 1 discloses an example in which, in an environment where a delay spread could exceed a guard interval corresponding to the CP length, Fourier transform is performed on a received signal between a window start timing and a window end timing that cover all received OFDM symbols that have passed multipaths.
FIG. 14 is a simplified block diagram illustrating a configuration of the receiving apparatus disclosed in NPL 1, the configuration relating to Fourier transform window timing determination. The receiving apparatus disclosed in NPL 1 in FIG. 14 includes a window start timing determination unit 1101 and a window end timing determination unit 1102.
The window start timing determination unit 1101 refers to a delay profile and outputs the first path as a Fourier transform window start timing.
The window end timing determination unit 1102 refers to the delay profile and outputs the end of an OFDM symbol of the last path as a Fourier transform window end timing.
FIG. 15 schematically illustrates Fourier transform window start and end timings in NPL 1. In FIG. 15, a) illustrates an FFT window start timing in conventional OFDM, and b) illustrates Fourier transform window start and end timings in NPL 1. As illustrated in b), according to NPL 1, Fourier transform window start and end timings are determined such that all OFDM symbols that have passed multipaths are covered. In this way, the maximum signal power of desired OFDM symbols can be recovered. According to NPL 1, while the interference component in a Fourier transform window increases as the length of the Fourier transform window extends, an interference canceller aims to remove the interference component from the received signal.    PTL 1: Japanese Patent Kokai Publication No. JP2005-168000A    PTL 2: Japanese Patent Kokai Publication No. JP2010-103716A    NPL 1: S. Suyama, H. Suzuki, K. Fukawa, “A MIMO-OFDM Receiver Employing the Low-Complexity Turbo Equalization in Multipath Environments with Delay Difference Greater than the Guard Interval,” IEICE Trans. Commun., vol. E88-B, no. 1, January 2005.